Datasheet

AMC7823
www.ti.com SLAS453F –APRIL 2005–REVISED MARCH 2012
When an external reference is used and SREF is set to '1', GREF has no impact on the reference.
However, if SREF is cleared to '0', a 10kΩ resistor is connected between pin 21, EXT_REF_IN,
and one of the two internal reference values dictated by the value of GREF. In this case, the
external reference must be able to drive the 10kΩ load. The full-scale range of the ADC input is
equal to 2 x V
REF
. To avoid ADC input saturation, GREF must be cleared to '0' when AV
DD
is less
than +5V and the internal reference is used. After power-on or reset, GREF is cleared to '0'.
Table 9 specifies the ADC input range as a function of bits GREF and SREF.
Table 9. Reference and ADC Input Range Configuration
BIT GREF BIT SREF REFERENCE ADC INPUT RANGE
0 0 Internal reference, 1.25V 0 to 2.5V
1 0 Internal reference, 2.5V, AV
DD
= 5V only 0 to 5V
Don't care 1 External reference V
REF
. 0 to 2 × V
REF
2 × V
REF
must not be greater than AV
DD
.
Set SREF = '1' when applying external reference.
ECNVT Enable CONVERT (external conversion trigger). This bit specifies the ADC trigger mode. When
ECNVT = '1', CONVERT is enabled. The ADC is in external trigger mode. The low-to-high
transition of the external trigger signal CONVERT triggers the ADC conversions. A write command
to the ADC Control Register does not initiate conversion, but rather specifies the group of inputs
to convert. After triggered by CONVERT, the AMC7823 sequentially accesses each analog input
one time. The bits [SA3:SA0] of the ADC Control Register comprise the channel address of the
first analog input accessed; [EA3:EA0] is the last analog input accessed. When the conversion
finishes, the ADC is idle and waits for a new CONVERT or a new command. With an external
trigger, the ADC always works in direct-mode (see the ADC Control Register section).
When ECNVT = '0', CONVERT is disabled. The internal ADC trigger is used. A write command to
the ADC Control Register generates the internal trigger and initiates ADC conversion. With an
internal trigger, the ADC can work in either direct-mode or auto-mode (see the ADC Operation
and ADC Control Register sections). Table 10 summarizes the ADC conversion mode
configuration.
After power-on or reset, DAVF, SREF, GREF, and ECNVT are cleared to '0'; RSTC is set to '1'.
Table 10. ADC Conversion Mode Configuration
ECNVT of AMC STATUS/ CMODE of ADC
CONFIGURATION REGISTER CONTROL REGISTER ADC CONVERSION MODE
1 – External Trigger, Direct-Mode
0 0 Internal Trigger, Direct-Mode
0 1 Internal Trigger, Auto-Mode
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